Full Idea
Among boots we distinguish left and right, so we can choose all the right or left boots; with socks no such principle suggests itself, and we cannot be sure, without the [Axiom of Choice], that there is a class consisting of one sock from each pair.
Gist of Idea
We can pick all the right or left boots, but socks need Choice to insure the representative class
Source
Bertrand Russell (Introduction to Mathematical Philosophy [1919], XII)
Book Reference
Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.126
A Reaction
A deservedly famous illustration of a rather tricky part of set theory.